It's spring planting time in Wisconsin, and I my Briggs & Stratton
powered rototiller lost all spark. I had to spring for what amounts
to a "new magneto".
Why wouldn't something like this work on a rotary/Wankel engine?
Two screws and a wire for the kill-switch is all that is required to
install this unit externally to the flywheel.
http://www.ebay.com/itm/Briggs-Stratton-Ignition-Coil-492341-495859-
NEW-/260635372094?pt=LH_DefaultDomain_0&hash=item3caf142a3e
Admittedly, my Briggs has its problems. The armature/flywheel magnet
air-gap is supposed to be 0.010, and my Briggs flywheel was probably
cast in China where some guy did not know the meaning of the term
"round" , so I have to run my engine with a considerably larger air-gap
to allow the flywheel to rotate all the way around (thereby
producing a weaker spark), but it does start/run quite well (without
a computer ):)
R Berglund
Good price for an outboard type of ignition.
It runs at 3600 RPM where the timing is fixed more or less.
What is the OD of the magnet/flywheel?
http://www.foxvalleykart.com/timing2.html
Paul Lamar
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How does the Magnetron Ignition System function? How can it be
tested? Ignition System Theory and Testing
Now let's look at the venerable Briggs & Stratton and its
"Magnetron" ignitions system. It seemed impossible to believe when
Briggs told us we could yank out the points and condenser that had
served us to well for years and plug that little box in their place.
Well, not only does it work, but also it works better than the old
points ever did. Here's how: The magnetron coil actually contains 3
separate coils of windings. In addition to the primary and secondary
windings, there is a 3rd winding, called the "trigger" coil." When
the leading edge of the magnets in the flywheel approach the coil,
the magnetic field surrounding those magnets generates a small
voltage (about 1 volt) that powers a solid-state switching device
called a Darlington Transistor, turning the transistor "on." That
"on" mode completes the circuit on the primary winding and a current
of about 3 amps flows to ground on the crankcase
(Figure 2). As the flywheel continues to rotate that current builds
the magnetic field in the secondary winding.
When the trailing edge of the magnets in the spark plug gap. the
flywheel reaches the trigger coil, a second small current is induced
in the trigger coil and that current tells the transistor to "turn
off", effectively breaking the circuit of the primary winding to
ground (Figure 3). As we discussed last month, the sudden break
causes the magnetic field to collapse. Since current (amperage) and
voltage are inversely proportional, suddenly stopping the current
flow causes the voltage to "spike" as it compensates. It's that
spike that builds the voltage required in the secondary winding to
jump the sparkplug gap and ignite the mixture. By the way, in the
Magnetron coil, the primary winding has 74 turns of wire and the
secondary has 4400. That's a 59.5:1 ratio and that helps insure that
there's enough voltage to get the job done. All this, the initial
current in the trigger coil, the buildup of potential, the second
trigger coil pulse, and the collapsing field with the resulting
voltage spike and spark, in on about 10 degrees of crank rotation.
That means that at 6000 RPM the whole thing, start to finish, only
takes about 0.00027 seconds. Pretty amazing.
This simple, but very effective, spark control system even advances
the ignition timing, causing the plug to fire earlier (in degrees of
crank rotation) as RPMs go up. You may recall from the part on of
this series (NKN January,2000) that "the rate at which (the)
potential develops determines its magnitude." In other words, the
faster you spin the magnet on the flywheel past the windings in the
coil, the greater the potential voltage that is developed. Since it
only takes about 1.0 volts from the trigger coil to turn the
Darlington transistor on (and off), the faster (in terms of crank
rotation degrees) the potential builds to the required 1.0 volts,
the earlier the induced current in the primary side of the coil
will begin to build. And likewise, the earlier the second trigger
coil potential reaches 1.0 volts, the earlier it will turn the
transistor off and trigger the spark process. Take a look at Figure
3 to see how this advances the timing. I know this sounds like a lot
of engineering gobbledygook. But what it means to you as a racer is,
the faster you turn the engine, the earlier the spark occurs in the
rotation of the crank. "Advancing" the spark is something that
engine tuners from karts to NASCAR have played with for years. Most
big-car ignition systems have had built-in ignition advance for
decades. In more recent years, old mechanical systems to change the
timing in the distributor have been replaced with sophisticated
electronic engine control systems. The Briggs "Magnetron" system
does exactly that, cleanly, efficiently, and reliably.
Probably the most commonly used ignition-tuning tool on the Briggs &
Stratton 4stroke is the offset flywheel key. Every kart shop carries
them and the savvy Briggs racer carries a selection to adjust the
engine to the track. Basically, these little machined pieces replace
the stock Briggs 1/8" X 1/8" flywheel key. Although they are
sometimes marked in thousandths of an inch offset, more commonly
they are identified by degrees of offset. In other words, how many
degrees of crank rotation do they move the flywheel from the stock
position. Most engine builders building methanol-burning Stock-class
engines usually send their engines out the door with 5 degrees or so
of offset. That amount of advance provides a reasonably safe
performance increase without too many headaches for the less
experienced or adventurous tuner. Part of that improved performance
comes from starting the combustion process itself earlier and thus
optimizing the point at which the combustion chamber pressure
reaches its peak. Another part comes from the fact that the
methanol/air mixture burns somewhat more slowly than a gasoline/air
mixture for which the engine was designed. So you gas-classers out
there take note; 2 or 3 degrees is a better place for you to start.
The 1980's ushered in the magic black box, [WINDOWS-1252?]Magnetron™
ignition coil,
Briggs & Stratton's first truly electronic ignition system. Breaker
point ignition systems for most small air-cooled engine
manufacturers have totally disappeared over the last 20 years. In a
point style system, the flywheel magnets rotate past the legs of the
ignition armature. The armature itself is made up of two separate
windings of copper wire - the primary and secondary - one wound on
top of the other. Basic physics tells us that when a magnetic field
(flywheel magnet) cuts through (moves past) a conductor
(copper wire), a flow of electrons (electricity) is created. However,
electron flow only occurs when we have a complete circuit. This
means that the points must be closed. We're also told that the
faster the movement between the field and the conductor, the greater
the output.
Remember science class in grade school? At one point in your school
career, an enterprising teacher wrapped a length of copper wire
around a nail and hooked the wire ends to a dry cell battery. A
handful of paper clips was instantly attracted to the nail and fell
away when the battery was disconnected. Electron flow through a
conductor then, causes a magnetic field. When the points close,
electron flow causes a magnetic field to be created around the
primary. This field also envelopes the secondary. The points now
open, break the circuit and collapse the field through the primary.
The field caving in on itself is movement just like the rotating magnetic
field of the flywheel. This movement is at speeds much greater than
the flywheel could spin - near the speed of light. The rapidly
collapsing field tears through the secondary winding which has sixty
turns of wire for every one turn of the primary - effectively
generating 60 times the voltage created in the primary. The sum
total of this is that the secondary winding can create up to 25,000
volts in some systems, which is used to jump the air gap of the
spark plug and ignite the fuel/air mixture in the combustion chamber.
Now for the black box. [WINDOWS-1252?]Magnetron™ solid state ignition
systems, in
essence, replace the mechanical breaker points with a transistor.
That is, we replace a mechanical switch with an electronic one. No
moving parts, no arcing, no adjustments and solid state reliability.
Remember speed is a factor. The engine must be pulled over at a
minimum speed of 250 RPM before the coil will even think about
firing. Thick oil on a winter day or a heavy parasitic load may
cause problems. Customers come into play here as well. Shorter or
elderly individuals may not have the leverage or strength required
to reach the RPM required to activate the Magnetron's electronics.
The tester electrode gap is .166" wide. Those wise in the way of
electrons have calculated that it takes around 13,000 volts to jump
this gap. We need 10,000 to jump the gap on a cold spark plug.
When under compression, the plug requires twice the voltage to fire.
Briggs & Stratton air gaps cannot be made too wide to prevent spark
providing the coil is healthy and the engine is spun over fast
enough. A wide air gap, say .030" will ever so slightly retard the
ignition timing as the magnetic field takes longer to build within
the coil windings.
B&S
Years ago I had the weapon 1 and an old B&S powered lawn mower. Something
happened to the ignition and it would not run. Had it apart a few times. I
found I could make it run with an old 6V coil and a lantern battery with the
points to trigger it. The only problem was when weapon 1 mowed the lawn. She
could not get used to the idea to remove the + wire she had to connect to
start it when done. She would just kick the plug ground over and it quit. Next
time I went to mow the battery was dead. No training the girl!
One day a sales man came into the service station and divulged he had an
electronic ignition module for lawn mowers. Just out. I bought one for this
old mower (had tried to get the original but told it was obsolete). When it
came, I installed it with great anticipation only to be let down. It did not
work so I removed it to give it back. He did not want to give my money back
but sent another which did work. I think I still have the coil around
somewhere. Saved on batteries and temper! No points to ever adjust either.
Dale Davies
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